I'd get 18" twists, but anything up to 22"will actually work. The faster twists are better for heavier bullets for long range is all and a double would't be used for that.
; You will be safer in the accuracy deparment with 18" to 20" of twist.
; Check www.trackofthewolf.com for liner diameters. What gauge is the shotgun? They have liners of various diamters, sold by the inch. They also have barrels of various sizes and lengths. The Green Mountain barrels are accurate, well made and won't break the bank.
; To keep the weight down, swamping the liners might be a consideration as was done by a guy on this site some time ago. Perhaps you can do a search for the excellent pictures and methods they used?